The Technique of Microcalorimetric Adsorption and Its Application in Characterizing Surface Acidity/Basicity of Metal Oxides

The technique of microcalorimetric adsorption and its application for measurements of surface acidity and basicity have been reviewed. Specifically, the strength and number of surface acid and base sites can be quantitatively titrated by the microcalorimetric adsorption of NH 3 and CO 2 as probe molecules. In addition, in situ infrared spectroscopy provides the information about the nature of the surface acid and base sites. For example, Lewis acid and base sites on metal oxides are coordinatively unsaturated metal cations and oxygen anions, respectively, while Brnsted acid and base sites are the hydroxyl groups on the surface. In addition, the strengths of surface acidity and basicity as defined by the adsorption heat of NH 3 and CO 2 can be correlated with Sanderson electronegativity of the metal oxides, and the interactions of basic probe molecules with acidic surface may be described by Drago parameters. Thus, strong surface acidity can be obtained by preparing the surface possessing coordinatively unsaturated metal cations with high electronegativity while strong surface basicity may be obtained by preparing the surface possessing coordinatively unsaturated oxygen anions associated with metal cations with low electronegativity.